Method and apparatus for making directional solidification castings

ABSTRACT

A method and apparatus for producing large, directionally solidified cast articles having a cast height greater than 200 mm, and preferably greater than 300 mm is described. The casting mold reliability, as well as the casting equipment, is improved by proportioned pouring of a molten superalloy into a casting mold where the mold is protected by suspender elements. The cast articles made by the inventive method are of particular interest to aircraft and power generation equipment, and include, but are not limited to, components such as blades, airfoils, buckets, nozzles, and the like.

TECHNICAL FIELD

[0001] This invention relates to a method and apparatus for making directionally solidified castings, and the articles made therefrom. More particularly, the invention relates to making parts greater than 200 mm in length for gas turbine engines. The invention further relates to a method and apparatus that provide improved reliability of the ceramic molds used in the casting process for the articles.

BACKGROUND OF THE INVENTION

[0002] It is known that the properties of superalloys can be improved by directional casting techniques that produce unidirectional columnar grained or single crystalline articles. Single crystalline articles differ from conventional polycrystalline articles primarily by the absence of boundaries between differently and arbitrarily oriented crystals. Presently, industries that utilize superalloy cast articles, such as the aircraft and land-based gas turbine businesses, desire to have the option of single crystal or polycrystalline directionally solidified cast articles.

[0003] Directionally solidified castings, including single crystal and polycrystalline, are manufactured by a unidirectional solidification process in which a casting shell mold containing molten superalloy material is withdrawn downward from a heated furnace. The molten superalloy is solidified gradually from the lower end of the casting mold to the upper end. For single crystal castings, a crystal seed with a crystallographic direction, such as <011>, is planted in the base of the casting mold.

[0004] One process of manufacturing directionally solidified castings comprises the pouring of molten metal into a mold within a heated zone, with the base of the mold being cooled by a chill plate, and subsequent crystallization of the molten metal occurring by gradually withdrawing the mold from the heated zone so that the bottom of the mold and upward are cooled by convective radiation to solidify the cast article. This process is discussed in U.S. Pat. No. 3,857,436. Another process for making directionally solidified cast articles comprises pouring molten metal into a superheated mold situated in a heated zone and withdrawing the mold from the furnace into a liquid coolant bath, where the coolant bath has a temperature lower than the solidus temperature of the cast metal. This process is described in U.S. Pat. No. 3,915,761. Additional variants of these methods are described in Russian Federation Patent N 2010672 and Russian Author Certificate USSR N 1061926. In another process U.S. Pat. No. 5,309,976 teaches partially filling a mold with a metallic melt within a casing chamber or furnace and the remaining melt is poured into the mold at a rate less than the first mold fill as a solidification front propagates through the melt. The second pour rate corresponds to the melt solidification rate. The process of U.S. Pat. No. 5,309,976 is limited to initial pouring rates and secondary pouring rates to help prevent mold failure.

[0005] The above-mentioned methods are deficient in that the cast articles which are produced have a limited size length and cross-section. This is due in part to the unreliability of the casting mold during the casting process. For large casting molds, say greater than 200 mm in length, hydrostatic forces act on the casting mold when molten alloy is poured into the mold and the mold is subsequently cooled. These forces tend to warp the contours of the casting mold and the resultant cast article. The molds themselves often break apart during the casting of large articles. U.S. Pat. No. 3,927,710 teaches that a mold may be supported by ceramic rings. Thus, there is a need for a method and apparatus to produce large size cast articles, greater than about 200 mm in length, where the casting mold is reliable throughout the casting process.

SUMMARY OF THE INVENTION

[0006] This invention satisfies the above-mentioned need by providing a method of making directionally solidified cast articles, distinguished by having improved reliability of a casting mold, said method comprising the steps of: heating the casting mold to a predetermined temperature in a heating zone in a casting furnace; pouring molten superalloy into the casting mold in the heating zone in an initial amount sufficient to maintain a level of the molten superalloy above a solidification front of the cast article to prevent disturbance of a crystalline growth of the directionally solidified cast article; simultaneously directionally solidifying the molten superalloy in the casting mold by withdrawing the mold from the heating zone into a cooling zone while further pouring of the molten superalloy into the casting mold at a rate to maintain the level of the molten superalloy above the solidification front; and finishing the pouring of the molten superalloy into the casting mold so that a height of the solidified portion of the cast article is greater than one half of an overall height of the cast article.

[0007] In another aspect of the invention there is provided a suspender system to improve the reliability of the mold during the casting process. The suspender system is comprised of horizontal load-bearing beams or rods which closely embrace the casting mold in the casting furnace. Another embodiment of the suspender system includes incorporating the beams or rods into the manufacture of the mold itself. This is accomplished by making a pattern from wax or resinous material for the shape of the mold.

[0008] In yet another aspect of the invention, there is provide a pouring riser that is attached to the mold along the height of the mold's vertical side. The pouring riser has at least one passageway, and preferably more than one passageway into the interior cavity of the mold, with a stationary pouring cup telescopically connected to the pouring riser. The pouring cup is located near the top of the furnace.

[0009] The following drawings and detailed description further describe the invention.

BRIEF DESCRIPTION OF THE DRAWINGS

[0010]FIG. 1 is a schematic view of a ceramic shell mold for a cast article blade with its base upward in a suspender system.

[0011]FIG. 2 is a schematic view of a ceramic shell mold for a cast article blade with its base downward in a suspender system.

[0012]FIG. 3 is a schematic side view of the ceramic shell mold for a cast article blade from points A to A of FIGS. 1 and 2.

[0013]FIG. 4 is a schematic view of a ceramic shell mold with a pouring riser, at least one passageway into the cavity of the mold, and a stationary pouring cup.

[0014]FIG. 5 is a schematic view of the casting pattern of the shell mold, the skeleton design of the mold with the suspender system incorporated in the mold pattern, the flanges of the suspender system and the starting zone in the mold for the onset of the directional solidification of the molten superalloy.

DETAILED DESCRIPTION OF THE INVENTION

[0015] The instant invention is directed towards a method and apparatus for producing large, directionally solidified cast articles having a cast height greater than 200 mm, and preferably greater than 300 mm. As a result of the invention, the casting mold reliability, as well as the casting equipment, is improved. The cast articles made by the inventive method are of particular interest to aircraft and power generation equipment, and include, but are not limited to, components such as blades, airfoils, buckets, nozzles, and the like.

[0016] In general terms, the invention can be described as heating a mold, shell, shell mold, or casting mold, herein all terms mean mold, which usually contains a ceramic material, to a predetermined temperature in a heating zone in a casting furnace. The predetermined temperature is a temperature of the mold that is sufficient to accept the molten superalloy material before crystallization or solidification of the superalloy begins. A preheat temperature for the mold that is above the melting temperature of the superalloy is recommended. The casting furnace atmosphere is generally maintained under vacuum conditions. The mold may have a starter crystal seed in its cavity to initiate the growth of a desired crystallographic oriented single crystal article. As a result, the mold can be designed for polycrystalline or single crystal cast articles.

[0017] After heating the mold to a predetermined temperature, the molten superalloy (herein also referred to as melt) is poured into the mold. The superalloy metal may be heated and melted in a separate furnace or in a separate compartment of the casting furnace. The temperature of the molten superalloy (melt) at the instant of its pouring into the mold may be about 100° to 120° C. higher than the temperature corresponding to the beginning of crystallization, but somewhat lower than the temperature of the mold.

[0018] The method of the invention is further characterized in that the directional solidification of the molten superalloy starts after a sufficient amount of molten superalloy has been poured into the mold so that a level or height of the molten superalloy above the solidification front is adequate to prevent turbulence or disturbance of the crystal formation during the directional solidification process. A sufficient initial amount of molten superalloy that is poured into the mold may be about 10 to 40% of the volume of the mold, and preferably, is about 20 to 30% of the mold volume. Once a sufficient amount of molten superalloy has been poured into the mold, directional solidification of the molten superalloy starts by withdrawing the mold from the heated zone into the cooling zone. The cooling zone may be a chill plate or liquid metal cooling bath, or other suitable cooling method. The invention herein will be described in terms of the cooling zone being a liquid metal cooling bath, such as liquid tin or aluminum. It is also noted that a baffle may exist between the heating and cooling zones to further increase the temperature gradient when directionally solidifying the cast article.

[0019] Upon pouring initially of a sufficient amount of the molten superalloy into the mold, such as about 20-30% by volume, thereafter, the further pouring of the molten superalloy into the mold is performed simultaneously with the directional solidification process through a poring cup in such a way that a required level of the molten superalloy would be provided above the solidification front of the cast superalloy metal to prevent disturbance of the crystal growth of the cast article.

[0020] It should be pointed out that if the initial volume of the molten superalloy that is poured into the mold is not a sufficient amount prior to the start of the solidification process, than parasitic grain growth in various crystallographic directions may take place. Thus, one skilled in the art would be able to determine a desirable initial amount of molten superalloy metal to pour into the mold to promote single crystal or polycrystalline unidirectional solidification of the cast article. As previously stated, about 20-30% by volume of the mold is preferred for the initial amount of poured molten superalloy.

[0021] In this invention, solidification takes place simultaneously with the proportioned pouring of the molten superalloy at a rate which provides the progressive raising of the molten superalloy level above the solidification front in such a way that when the melt pouring finishes, the height of the solidified portion of the cast article should be greater than about one half (½) of the total casting height of the article. This also serves to decrease the mechanical load on the ceramic mold. In order to produce an acceptable casting of high quality, the molten superalloy level above the solidification front should be a sufficient height so that turbulence does not effect the solidification of the cast article. A suggested height may be about 30 to 70 mm. Again, one skilled in the art may determine without undue experimentation the appropriate height level depending on the mold size and shape.

[0022] Another element of this invention which also increases the reliability of the mold, is a special suspender system. The shell mold may be positioned into the suspender system in the casting furnace prior to the start of the casting process or the suspender system may be made a part of the actual shell mold during the pattern stage of making a shell mold. In the first instance, the suspender system comprises suspender components or elements which comprise horizontal load-bearing beams that closely embrace or surround the shell mold at points or positions spaced about ¼ section of the mold height. The beams bear the hydrostatic pressure of the molten superalloy when it is poured into the mold. The beams or rods are made of molybdenum, graphite, a graphite-based composite material, or mixtures thereof. Referring to FIG. 1, the shell mold 1 is suspended on two upper horizontal load-bearing beams 2 which are positioned in the openings of the upper graphite inserts 3. The next pair of horizontal beams 4 are then positioned in place. The beams closely embrace the mold side surfaces and they are fastened with wedges of graphite, molybdenum, graphite-based composites, or mixtures thereof, to provide a fixed position of the shell mold in the suspender system. The amount of beams that are used depends upon the height, width and cross-sectional dimensions of the mold. For instance, the horizontal beams should be positioned at about every 100±50 mm when the width of the casting is <200 mm. One skilled in the art can use standard mechanical principles to adjust the amount and spacing of the beams to embrace and support the mold during the casting operation. This suspender system is hung within the casting furnace by two vertical hangers made of molybdenum, graphite, a graphite-based composite material, or mixtures thereof. The mold then fits within the suspender system. In FIG. 2, the shell mold 5 is suspended on two upper horizontal load-bearing beams 6 which are positioned in the openings of the upper graphite inserts 7. The next pair of horizontal beams 8 are then positioned in place. FIG. 3 is a cross-sectional view of FIGS. 1 and 2. The shell mold is 9 is suspended on two upper horizontal load-bearing beams 10 which are positioned in the openings of the upper graphite inserts 11. The next pair of horizontal beams 12 are then positioned in place.

[0023] In another embodiment of the suspender system, there is provided suspender components or elements, such as beams, bars, or rods, incorporated as part of the mold itself. The suspender components or elements are positioned in the casting pattern of the mold on the exterior of the mold. The mold is first made from a wax or plastic pattern, which later receives a ceramic slurry coating to eventually form the ceramic mold for casting of articles. In the initial stages of forming the wax or plastic pattern, the horizontal beams and vertical support beams are placed as a skeleton in the casting pattern for the mold. The skeleton is shaped for instance with cylindrical members with flanges on their butt ends on the outer surface or exterior of the mold. The skeleton section for the suspender elements form exterior cavities on the mold. Once the mold is produced according to methods known in the art, such as slurry casting, the suspender components, the molybdenum, graphite, graphite-based composite, or mixtures thereof, rods or beams, are placed inside the skeleton on the mold and the butt ends of the skeleton are sealed with a ceramic slurry. The skeleton members are positioned above the starting zone on the mold and are distributed uniformly along the entire height of the mold. The mold is then positioned on a hanger in the casting furnace to be heated to a predetermined temperature as mentioned above in the method of this invention. FIG. 5 shows the casting pattern 19 and the pattern of the skeleton 20 with flanges 21 and the starting zone 22.

[0024] In addition to the suspender elements of this invention, there is still provided other aspects of the invention which include the pouring cup and the pouring riser. A pouring cup is placed into the mold in the heating zone of the casting furnace. In order to eliminate sputtering of the molten superalloy upon its pouring into the mold, the pouring cup has an elongated side portion which enters a base of the mold, if there are no inner cores within the mold. If inner cores are present in the mold, then the cup enters a special pouring channel. In another embodiment of the invention, to reduce the turbulence of the pouring of the molten superalloy above the solidification line, a pouring riser is positioned to one lateral side of the mold. This is done in the casting pattern for the mold. The pouring riser is connected along the entire height of the pattern and with the casting cavity by the passageways inclined at the angles of about −70 to +70 to the horizontal plane. The cross-section of each passageway may be about 1-3 mm if they also play the role of filtration when the molten superalloy is poured into the mold. Thus, when a pattern for a mold is made, both the pouring riser and the suspender system may be incorporated into the pattern and subsequently, become part of the final mold. Referring to FIG. 4, the ceramic shell mold 13 has a pouring riser 14 positioned on the lateral side of the mold with passageways 15 of small cross-section. The pouring cup 16 is stationary and is place near the top of the furnace. The stationary cup 17 for simultaneous pouring of the molten superalloy into two clusters of molds is also shown.

[0025] In the invention the molten superalloy is poured into the mold through a stationary cup located at the top section of the furnace. The cup telescopically enters the pouring riser. When it is desired to pour the molten superalloy into two or more mold clusters simultaneously, then a stationary pouring cup is used which has a pouring member for each pouring riser.

[0026] The molten superalloy pouring into the mold cavity from a riser through the passageways which are inclined at proper angles to the horizontal plane, as described above, provides smooth movement of the molten superalloy without any turbulent stream which may cause the parasitic grain growth in the cast article. Further, the passageways that have small enough cross-section, help to eliminate the formation of nonmetallic inclusions in the cast article.

[0027] The following examples further serve to demonstrate, but not limit, the scope of the invention.

EXAMPLE 1

[0028] A casting furnace with a liquid coolant bath (Russian commercial model unit UVNK-8P) was used to make large castings in accordance with the present invention. The shell mold was for a large-sized airfoil having dimensions of height equal to 400 mm, chord or cross-section of 180 mm, and the width of the base being 200 mm. The shell mold had a thickness of about 8-10 mm and was solidified with its base upward. The mold had a single crystal seed in a starting cavity and was positioned inside a special suspender, as shown in FIGS. 1 and 3. The suspender system included two vertical molybdenum hangers consisting of rods with a diameter of 20 mm and interconnected by graphite inserts (wedges) having two openings. The distance between the openings is equal to about the shell mold's thickness.

[0029] In order to eliminate the mold distortion under the influence of its own weight and the molten superalloy hydrostatic pressure, the load is transferred to the horizontal beams of the suspender. These beams closely embrace the mold along its perimeter and effectively prevent its distortion. The horizontal load-bearing beams are made of molybdenum and have a cross-section of 10 times 20 mm.

[0030] The pouring cup has a gauged hole and is placed inside the mole cavity in such a way that the pouring hole is positioned at the level of about 200 mm from the mold's upper edge.

[0031] The corrosion resistant nickel-based superalloy, of the Russian type ZhSKS, was melted in an induction furnace having the crucible volume of about 20-25 kilograms. The mold was preheated to the temperature of about 80-100° C. higher than the melting temperature of the superalloy. The temperature of the superalloy in the crucible was raised to about 1560±20° C. and the molten superalloy was poured into the mold through a pouring cup. Once about 4-5 kilograms (about 20% by volume of the mold) of the molten superalloy had been poured into the mold, the directional solidification process was initiated by lowering the mold from a heating zone into a cooling zone, while the molten superalloy was simultaneously poured from the inductor into the mold as the mold was withdrawn into the cooling zone.

[0032] At the time when the molten superalloy was finished being poured into the mold, about one half of the mold had already been lowered into the cooling zone. The mold continued to be lowered into the cooling zone with the superalloy until the mold was completely immersed in the cooling zone. The solidified casting was removed from the suspender and from the ceramic mold to reveal the macrostructure of the cast article. It was found that the use of a single crystal seed in the starting cone, the crystal orientation selection, and the directional solidification process provided a cast article having a single crystal structure of a desired orientation along the entire height of the casting.

EXAMPLE 2

[0033] In this example the airfoil was solidified with its base downward. The method of its position in the suspender system is shown in FIG. 2. The horizontal load-bearing beams were made of sintered graphite. In this example, the directional solidification process was started after about 30% of the volume of the mold (about 7-7.5 kilograms of molten superalloy) was filled with the nickel-base superalloy, Russian type ZhS. The method of proportioning the pouring of the molten superalloy and the conditions of the directional solidification process, are as describe in Example 1. The large size airfoil casting thus produced had a single crystal structure along the entire height.

[0034] The use of the proportioned pouring of the molten superalloy with a simultaneous directional solidification process and also the use of a special mold suspender system provide the improved reliability of the shell mold and of the casting equipment. The efficiency of the directional solidification process is also improved and enables the production of quality, large airfoil and blade castings, greater than 200 mm in height, and preferably greater than 300 mm in height. This applies to single crystal and polycrystalline cast articles.

EXAMPLE 3

[0035] This example is further illustrative of the present invention. To one of the lateral sides of the large airfoil (blade) pattern there was positioned a pouring riser which was interconnected with the casting cavity along the entire height by about eight (8) passageways of 2 mm in diameter, in which the passageways were inclined at the angle of about 70° to the horizontal plane. Then a skeleton for the suspender's components was prepared where the skeleton had flanges on the butt ends.

[0036] After the pattern had been produced, the ceramic shell was made according to a slurry dipping technique known in the art and was then fired at 1250° C. for about 4 hours. The molybdenum suspender components were positioned into the cooled mold inside the ceramic skeleton. The skeleton butt ends were then sealed with a ceramic slurry. The mold and suspender were placed in the casting furnace, Russian commercial model UVNK-8P, and the heating chamber was evacuated to 1×10⁻³ mm m.c. The nickel-base superalloy of Russian type ZhS was melted in an induction furnace. The molten superalloy was poured into the mold through the stationary cup via the inclined passageways. The stationary cup was telescopically inserted into the pouring riser. The pouring procedure was performed in two steps. First, about 20% by volume of the mold of the molten superalloy was poured into the mold, and solidification begun by lowering the mold into the coolant at the rate of 10 mm/min. After the solidification had begun in a starting zone of the mold, the pouring of the molten superalloy was continued, while the withdrawal of the mold from the heating zone into a cooling zone was continued. At the moment when the pouring of the molten superalloy was completed, the mold was withdrawn one half its height into the cooling zone. The mold with the molten superalloy had then been completely lowered until it was fully immersed into a liquid metal coolant bath.

[0037] After the solidification process had finished and the preheating furnace had been switched off, the mold was removed from the coolant, the solidified casting was removed from the suspender and from the ceramics, and the macrostructure of the cast article was revealed. The cast article was 450 mm in height and had the single crystal structure of [001] orientation alone the entire height. In the course of the directional solidification process, the ceramic mold was absolutely free of distortion. The suspender components were protected from the coolant metal bath.

[0038] This process was also repeated using passageways of about 3 mm diameter being inclined at the angle of about +70°. When the diameter of the passageways is more than 3 mm, the passageways do not play the role of filters and coarse nonmetallic inclusions can penetrate a casting. The passageways of the diameter less than 1 mm may be difficult to produce.

[0039] The inventive methods allow the casting of large articles, single crystal and polycrystalline, of required geometric dimensions. The suspender components are reusable in the directional solidification process. 

What is claimed:
 1. A method of making directionally solidified cast articles, distinguished by having improved reliability of a casting mold or molds, said method comprising the steps of: heating the casting mold to a predetermined temperature in a heating zone in a casting furnace; pouring molten superalloy into the casting mold in the heating zone in an initial amount sufficient to maintain a level of the molten superalloy above a solidification front of the cast article to prevent disturbance of crystalline growth of the directionally solidified cast article; simultaneously directionally solidifying the molten superalloy by withdrawing the mold from the heating zone into a cooling zone while further pouring of the molten superalloy into the casting mold at a rate to maintain the level of the molten superalloy above the solidification front; and finishing the pouring of the molten superalloy into the casting mold so that a height of the solidified portion of the cast article is greater than one half of an overall height of the cast article.
 2. A method according to claim 1 where the directionally solidified cast articles are single crystal or polycrystalline articles.
 3. A method according to claim 1 where the casting mold is heated above a melting temperature of the molten superalloy.
 4. A method according to claim 1 where the initial amount of the molten superalloy poured into the mold is about 20-30% of the volume of the mold.
 5. A method according to claim 1 where the level of the molten superalloy above a solidification front is about 30-70 mm in height.
 6. A method according to claim 1 where the cooling zone comprises a liquid metal bath.
 7. A method according to claim 1 where the cooling zone comprises a chill plate and a space without auxiliary heating.
 8. A method according to claim 1 where the casting mold comprises more than one interior cavity to make plural cast articles.
 9. A method according to claim 1 where the casting mold hangs vertically within the furnace, and said mold is embraced by a suspender system.
 10. A method according to claim 9 where the suspender system comprises horizontal and vertical suspending elements.
 11. A method according to claim 10 where suspending elements are made from a material selected from the group consisting of molybdenum, graphite, graphite-based composites, and mixtures thereof.
 12. A method according to claim 1 where the casting mold is made from a pattern having a suspender skeleton.
 13. A method according to claim 12 where the suspender skeleton has suspender elements placed into outer mold cavities and ends of said suspending elements are sealed with a ceramic slurry.
 14. A method according to claim 1 where said mold has a pouring riser attached on its lateral side with at least one passageway entering the interior cavity of the mold.
 15. A method according to claim 14 where said mold has more than one interior cavity and each cavity at least one passageway from the pouring riser entering the interior cavity.
 16. A method according to claim 15 where said passageway has an angle between about +70° to −70° to a horizontal plane.
 17. A method according to claim 16 where the passageway has a diameter of about at least 1 millimeter.
 18. A method according to claim 14 where the pouring riser has a stationary pouring cup that telescopically enters the pouring riser.
 19. An article made according to the method of claim 1 .
 20. An article according to claim 14 being a single crystal.
 21. An apparatus for casting directional solidified articles comprising: an induction furnace for heating metal, a heating means connected to the induction furnace to heat the metal; a chamber for preheating a casting mold; a heating means to preheat the mold and chamber; a pouring means in the induction furnace to pour the heated metal into the preheated mold; a mold suspender means in the chamber for preheating the mold; a cooling chamber; and a means to maintain a controlled atmosphere in each chamber.
 22. A casting mold having an interior casting cavity with dimensions for a cast article and exterior cavities for suspender elements. 